Diffused junction photodetector and fabrication technique

ABSTRACT

A diffused junction semiconductor ( 12 ) for detecting light ( 48 ) at a predetermined wavelength is provided including a base ( 30 ) and an epitaxial structure ( 32 ) electrically coupled to the base ( 30 ). The epitaxial structure ( 32 ) forms a p-n junction ( 38 ) in the base ( 30 ). The epitaxial structure ( 32 ) includes at least one diffusion layer ( 50 ) electrically coupled to the base ( 30 ). At least one of the diffusion layers ( 50 ) contributes impurities in at least a portion of the base ( 30 ) to form the p-n junction ( 38 ) during growth of the epitaxial structure ( 32 ). A method for performing the same is also provided.

RELATED APPLICATION

The present application is a continuation-in-part (CIP) application ofpublished U.S. patent application Ser. No. 09/976,508 filed Oct. 12,2001, entitled “WIDE-BANDGAP, LATICE-MISMATCHED WINDOW LAYER FOR A SOLARENERGY CONVERSION DEVICE”, Pub. No. US 2003/0070707 A1, which isincorporated by reference herein.

TECHNICAL FIELD

The present invention relates generally to telecommunicationtransceivers, and more particularly, to a diffused junctionphotodetector for use in a transceiver and a method of fabricating thesame.

BACKGROUND OF THE INVENTION

Telecommunication transceivers are utilized in various applications totransmit and receive communication signals in telecommunicationnetworks. Fiberoptics are used as a transmission medium between thetelecommunication transceivers for various transmission reasonsincluding low noise interference, high-speed data transmission rates,and large multiplexing capabilities. In order for the telecommunicationtransceivers to receive the communication signals transmitted via lightover fiberoptic cable, photodetectors are utilized.

Photodetectors transform light energy into electrical energy. Reversesaturation current is controlled by light intensity that shines on thephotodetectors. The light generates electron-hole pairs, which inducecurrent. The resulting current is directly proportional to the lightintensity.

The use of fiberoptics introduces practical, feasible, and functionalrequirements. The photodetectors are preferably semiconductor diodesthat are inexpensive due to large quantity requirements, reliable, andcapable of responding to light signals having wavelengths between 1300nm and 1600 nm. It is also desirable for the photodetectors to providelow noise or low dark current and be amendable to high volumeproduction.

Commonly used photodetectors are industrially produced and include agermanium (Ge) base or Ge substrate. After formation of the Gesubstrate, two general processes are used to form a p-n junction. Thefirst process includes implanting phosphorus or arsenic impurities intothe Ge substrate and the second process includes growing epitaxialcrystal on the Ge substrate or by implanting phosphorus or arsenicimpurities into the Ge substrate as to create a p-n junction. Theabove-mentioned processes, as known in the art, are expensive, timeconsuming, complex, and have a high defect rate. The Ge substrate postformation doping of impurities to form a p-n junction is highlysusceptible to forming defects due to inherent nature of the postformation process.

It would therefore be desirable to develop a photodetector that provideslow noise and is capable of responding to wavelengths between 1300 nmand 1600 nm. It would also be desirable to develop a process forfabricating the desired photodetector that is inexpensive, less timeconsuming to produce, less complex, and has a low defect rate.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for a diffusedjunction photodetector for use in a transceiver and a method offabricating the same. A diffused junction semiconductor for detectinglight at a predetermined wavelength is provided including a base and anepitaxial structure electrically coupled to the base. The epitaxialstructure forms a p-n junction in the base. The epitaxial structureincludes at least one diffusion layer electrically coupled to the base.At least one of the diffusion layers contributes impurities in at leasta portion of the base to form the p-n junction during growth of theepitaxial structure. A method for performing the same is also provided.

One of several advantages of the present invention is the ability todiffuse impurities into the base during growth of the epitaxialstructure. In so doing, a p-n junction may be formed within asemiconductor using a relatively inexpensive technique and in arelatively short period of time as compared with traditional techniques.The inexpensive technique includes use of large area substrates inconjunction with a multi-layer wafer production metal organic vaporphase epitaxy (MOVPE) reactor.

Another advantage of the present invention is the ability to provide asemiconductor with low leakage current due to passivation of a widebandgap semiconductor around the p-n junction, application of ananti-reflective coating over the epitaxial structure, and carrierconcentration in the base.

Furthermore, the present invention provides application versatility inthat resistivity of various layers of the substrate may be modified,thereby, adjusting various semiconductor parameters.

Other advantages and features of the present invention will becomeapparent when viewed in light of the detailed description of thepreferred embodiment when taken in conjunction with the attacheddrawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagrammatic view of a telecommunication networkutilizing diffused junction photodetectors formed in accordance with anembodiment of the present invention;

FIG. 2 is a cross-sectional view of a mesa diffused junctionphotodetector formed in accordance with an embodiment of the presentinvention;

FIG. 3 is a cross-sectional view of a planar diffused junctionphotodetector formed in accordance with an embodiment of the presentinvention;

FIG. 4 is a cross-sectional view of a planar diffused junctionphotodetector formed in accordance with an embodiment of the presentinvention;

FIG. 5 is a cross-sectional view of more than one planar diffusedjunction photodetector including a channel stop and formed in accordancewith an embodiment of the present invention;

FIG. 6 is a logic flow diagram illustrating a method of fabricating adiffused junction semiconductor in accordance with an embodiment of thepresent invention;

FIG. 7 is a plot of carrier concentration versus emitter layer depth fora diffused junction photodetector formed in accordance with anembodiment of the present invention;

FIG. 8 is a plot of current density versus voltage for a diffusedjunction photodetector formed in accordance with an embodiment of thepresent invention; and

FIG. 9 is a plot of capacitance versus voltage for a diffused junctionphotodetector formed in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In each of the following figures, the same reference numerals are usedto refer to the same components. While the present invention isdescribed with respect to a method and apparatus for a diffused junctionphotodetector for use in a transceiver and a method of fabricating thesame, the present invention may be adapted to be used in various systemsand applications including: vehicle systems, control systems,communication systems, semiconductor lasers, photodetectors,photodiodes, fiber optic receiver detectors, solar cells, or othersystems or applications that may utilize a diffused junctionsemiconductor.

In the following description, various operating parameters andcomponents are described for one constructed embodiment. These specificparameters and components are included as examples and are not meant tobe limiting.

Also, in the following description the term “semiconductor” may refer toany solid state device such as photodetectors, photodiodes, solar cells,or other solid state device known in the art.

Additionally, the present invention is applicable for low capacitancedevices as well as low leakage current devices. In general, lowcapacitance devices are high-speed devices used in data detection. Lowleakage current devices are not high-speed but are low power deviceswith high capacitance, which may be used, for example, as monitors todetect and correct laser power levels.

Referring now to FIG. 1, a block diagrammatic view of atelecommunication network 10 utilizing diffused junction photodetectors12 formed in accordance with an embodiment of the present invention, isshown. The network 10 includes a central station 14 in communicationwith multiple remote terminals 16 via fiber optic cable 18. The remoteterminals 16 may be a large distance from the central station 14,represented by breaks 20. The central station 14 and the remoteterminals 16 have transceivers 22 containing photodetectors 12 that arefabricated in accordance with methods described in detail below.

Referring now to FIG. 2 is a cross-sectional view of a mesa diffusedjunction photodetector 12′ formed in accordance with an embodiment ofthe present invention, is shown. The photodetector 12′ includes a base30 electrically coupled to an epitaxial structure 32. The base 30includes a base layer 36 and an emitter layer 34, in combination forminga p-n junction 38. The base layer 36 has a base layer top surface 39.The emitter layer 34 has an emitter layer top surface 40 upon which theepitaxial structure 32 is grown, and side walls 41. A passivationdielectric layer 42 is coupled to the base 30 and prevents carrierrecombination of at least a portion of the base 30. The epitaxialstructure 32 is coated with an anti-reflective layer 44 and iselectrically coupled to one or more contacts 46. As light 48 is receivedat various wavelengths through the anti-reflective layer 44 to the p-njunction 38 electrical energy is produced at the contacts 46.

Although, in a preferred embodiment of the present invention the base 30is formed from germanium (Ge), other similar materials known in the artmay be used. The emitter layer 34 is at least partially formed duringgrowth of the epitaxial structure 32. The emitter layer 34 containsimpurities such as phosphorus (P) and arsenic (As) as well as otherimpurities known in the art.

The epitaxial structure 32 includes a first diffusion layer ornucleation layer 50, a second diffusion layer or spacer layer 52, and athird diffusion layer or contact layer 54. The nucleation layer 50, thespacer layer 52, and the contact layer 54 contribute impurities that arediffused into the base 30 to form the n-type emitter layer 34 having ap-n junction depth D. FIG. 2 does not show the junction depth correctly.Please refer to the initial disclosure figure. The nucleation layer 50may be formed of indium gallium phosphide (InGaP), indium aluminumphosphide (InAIP), GaAlInP, or other nucleation layer materials known inthe art. The nucleation layer 50 couples the emitter layer 34 to thespacer layer 52. The spacer layer 52 may contain one or more layers andmay be formed from gallium arsenic (GaAs) or other similar materialknown in the art. The base 30 when formed of Ga and the spacer layer 52when formed of GaAs have mismatching lattice structures, which aredifficult to couple together directly, the nucleation layer 50 serves asa coupler between these two layers. The spacer layer 52 has opticalproperties that allow photons to penetrate to the p-n junction 38. Thecontact layer 54 may also be formed from GaAs or other similar materialknown in the art. The contact layer 54 provides electrical contactbetween the contacts 46 and the p-n junction 38.

Referring now to FIG. 3 a cross-sectional view of a planar diffusedjunction photodetector 12″ formed in accordance with an embodiment ofthe present invention is shown. The photodetector 12″ is similar to thephotodetector 12′ of FIG. 2, except that instead of the emitter layer 34protruding outward from the base layer 36 an emitter layer 34′ isrecessed within window 60 in a base layer 36′ where an emitter layer topsurface 40′ is approximately flush with a base layer top surface 39′.Also, a passivation dielectric layer 42′ is coupled to the base layer36′ and the epitaxial structure 32′.

Referring now to FIG. 4 a cross-sectional view of a planar diffusedjunction photodetector 12′″ formed in accordance with an embodiment ofthe present invention is shown. The photodetector 12′″ is similar to thephotodetector 12″ of FIG. 3, except that a passivation dielectric layer42″ is a continuous layer over the contact layer 54′ and the base 30.

Referring now to FIG. 5 a cross-sectional view of more than one planardiffused junction photodetectors 12′, are shown, including a channelstop 62 between each photodetector 12′. The channel stop 62 may be usedto separate photodetectors 12′ and prevent reverse bias breakdown atedges 64 of each photodetector 12′. In a preferred embodiment of thepresent invention the channel stop 62 is formed of heavily doped p-typematerial of approximately 5×10¹⁸ parts per cubic centimeter.

Referring now to FIG. 6, a logic flow diagram illustrating a method offabricating a diffused junction semiconductor, such as photodetectors 12and 12′ in accordance with an embodiment of the present invention, isshown.

In step 100, the bases (substrate) 30 and 30′ are formed. The substrates30 and 30′ are selected having a particular p-type doping level. Variousp-type doping level substrates may be selected. The p-type doping levelis selected by determining a desired resistivity level of the substrates30 and 30′. In so doing, the p-n junction depths 38 may be altered, andsubsequently other semi-conductor properties may also be altered byadjusting resistivity level of the substrates 30 and 30′, as shownbelow. A relationship between initial p-type doping level of the bases30 and 30′ and the p-n junction depths 38 are best illustrated by FIG.6.

Referring now also to FIG. 7 a plot of carrier concentration versus p-njunction depth D for diffused junction photodetectors 12 and 12′ formedin accordance with embodiments of the present invention, is shown. AP-diffusion profile curve 70 and an As-diffusion profile curve 72 areshown with intersections 74 at three different doping levels, low 76,moderate 78, and heavy 80. As the initial doping level of the bases 30and 30′ are increased the depths 38 of the emitter layers 34 and 34′decreases.

In general, low resistivity substrates produce high shunt resistancephotodetectors. On the other hand, high resistivity substrates producelow capacitance devices due to the p-n junction depth being largerbecause of low doping concentration of the substrate and the emitterlayer width being broader. As known in the art, since capacitance isinversely proportional to the emitter layer depth and emitter layerwidth, capacitance values of photodetectors that are constructed fromhigh resistivity substrates are lower than capacitance values ofphotodetectors constructed from low resistivity substrates.

Additionally, leakage current can be reduced by reducing the size of thep-n junction depths 38. Application of a bias voltage to a p-n junctionresults in a depletion region. A depletion region is a region that isdepleted of carriers from intentional dopants. The reduction of the p-njunction depths 38 for low leakage current applications results in ashallow, more abrupt, junction, which minimizes potential for adepletion region. For low capacitance semiconductor devices a deepjunction is preferred, therefore, high resistivity, low doped substratesare used.

In step 102, a determination is made as to whether the semiconductor isa mesa configuration or a planar configuration. When the semiconductorbeing fabricated is to have a mesa configuration upon forming the basesteps 104-114 are performed, otherwise for planar configurations steps116-124 are performed for one configuration and steps 104-108 and steps126-130 are performed for another configuration.

Referring now to FIGS. 2 and 6, in steps 104-114, a p-n junction 38 isgenerated for the mesa configuration.

In step 104, the nucleation layer 50 is applied to the top surface 40 togrow the epitaxial structure 32 on the base layer 36. The nucleationlayer 50 in a preferred embodiment of the present invention diffuses Patoms into the base 30 to form the emitter layer 34 as the epitaxialstructure 32 is grown.

In step 106, the spacer layer 52, which is low-doped and of n-typematerial, is applied to the nucleation layer 50. As stated above thespacer layer 52 may include multiple layers. The spacer layer 52 in apreferred embodiment of the present invention diffuses As atoms into thebase 30 to form the emitter layer 34 as the epitaxial structure 32 isgrown.

In step 108, the contact layer 54, which is highly doped and also ofn-type material, is applied to the spacer layer 52. The contact layer 54may also contribute impurities into the base 30 to form the emitterlayer 34.

In one preferred embodiment of the present invention the nucleationlayer 50 is approximately 100 Å thick, the space layer 52 isapproximately 0.5 μm thick, and the contact layer 54 is approximately2000 Å thick. In another preferred embodiment of the present inventionthe nucleation layer 50 is between 0.9 E17 cm³ and 3.0 E18 cm³ doped orslightly doped, the spacer layer 52 is between 2.0 E17 cm³ and 4.0 E17cm³ doped or moderately doped, and the contact layer 54 is between 7.0E17 cm³ and 9.0 E17 cm³ doped or heavily doped.

In step 110, semiconductor devices are mesa-etched from the bases 30 and30′ and epitaxial structures 32 and 32′. The mesa-etching is performedpreferably using a wet-etching technique that is known in the art.

In step 112, a dielectric passivating material, such as silicon dioxide,is deposited on the bases 30 and the side walls 41 preferably using alow temperature chemical vapor deposition process to form the dielectricpassivating layer 42. The dielectric passivating layer 42 preventscarrier recombination of the base layer 36 and the emitter layer 34.

In step 114, windows are etched through the dielectric passivatingmaterial. The mesa passivation dielectric thickness is different thanthe AR coating thickness, so for simplicity the passivating dielectricis removed from an optical area of the semiconductor and a fresh ARcoating is applied having an accurately controlled thickness.

Referring now to FIGS. 3 and 6, in steps 116-124, a p-n junction 38 isgenerated for a planar configuration. In step 116, the passivationdielectric layer 42 is applied to the top surfaces 39 and 40. Thepassivation dielectric layer 42 may be applied using a low temperaturechemical vapor deposition process. The passivation dielectric layer 42aids in preventing leakage current.

In step 118, a mask, preferably formed of silicon nitrate Si₃N₄, isapplied to the top surface 39′ to form the window 60 in the passivationdielectric layer 42′. Within the window 60 on the top surface 39′ theepitaxial structure 32′ is formed in steps 120-124.

In step 120, the nucleation layer 50′ is applied to the top surface 39′to grow an epitaxial structure 32′ on the base layer 36′, similar tostep 104 above.

In step 122, the spacer layer 52′, which is low-doped and of n-typematerial, is applied to the nucleation layer 50′, similar to step 106above.

In step 124, the contact layer 54′, which is highly doped and also ofn-type material, is applied to the spacer layer 52′, similar to step 108above. Upon completion of step 124 step 132 is performed.

Referring now to FIGS. 4 and 6, in step 126 the passivation dielectriclayer 42″ is deposited over the contact layer 54′ and the base 30.

In step 128, the contact layer 54′, the spacer layer 52′, and thenucleation layer 50″ are selectively etched. An area of the passivationdielectric layer 42″ is masked off, and etching extends below the p-njunction 38 to the base 30, to form the window 60.

In step 130, dopants are diffused, in the form of a heat treatment todrive the P-N junction 38 into the base 30. Although, by performingsteps 104-108 and steps 126-130 over performing steps 116-124 noepitaxial regrowth is performed, there is more processing involved.

In step 132, the anti-reflective coating 44 is deposited on theepitaxial structures 32 and 32′ using preferably a plasma-enhancedchemical vapor deposition process. The anti-reflective coating 44 alsoaids in preventing leakage current. The anti-reflective coating 44allows most wavelengths of light to pass through to the p-n junction 38,especially desired wavelengths of interest.

In step 134, metallization windows are patterned on the anti-reflectivecoating 44 to minimize the amount of reflective light.

In step 136, anti-reflective coating 44 is etched down to the contactlayer 54 to allow the metal contacts to abut the semiconductor surface.The anti-reflective coating is a high resistance insulator, thus isetched to allow current to flow in the semiconductor.

In step 138, n-type metallization of atoms, such as gold germanium(AuGe), nickel (Ni), and Au, are preferably deposited on exposed GaAsatoms of the contact layers 54 and 54′ using electron beam evaporation.

In step 140, back metallization with atoms such as, titanium (Ti) andgold (Au), are similarly deposited on the contact layers 54 and 54′. Theback metallization is preferably 150 Å of Ti and 3000 of Au Å inthickness.

In step 142, the photodetector is then sintered at approximately 450° C.for approximately 5 minutes.

The above-described steps are meant to be an illustrative example, thesteps may be performed synchronously or in a different order dependingupon the application. The steps may be applied for varioussemiconductors and with various materials depending upon theapplication.

By growing and processing identical photodetectors having substrates ofvarying resistivities, differences in characteristics of eachphotodetector are noted in Table 1 and FIGS. 8-9.

Referring now to Table 1, p-n junction depth and reverse breakdownvoltage parameters are shown as a function of Ge substrate doping. Asimpurity concentration increase the p-n junction depth and the magnitudeof the reverse breakdown voltage decreases.

TABLE 1 Junction Depth and Reverse Breakdown Voltage as a Function ofSubstrate Doping Substrate Doping p-n Junction Depth - Reverse BreakdownConcentration (cm³) Normalized Voltage (V) 5.00E+15 1 −40 1.00E+17 .32−4 8.50E+17 .2 −2 4.00E+18 .12 −1

Referring now to FIG. 8 a plot of current density versus voltage for adiffused junction photodetector, as a function of substrate doping,formed in accordance with an embodiment of the present invention isshown. Curve 82 represents a sample photodetector having a 2.5 E15doping concentration, 1.0 mm normalized p-n junction depth, and 1.7Kohms p-n junction resistance. Curve 84 represents a samplephotodetector having a 1.0E17 doping concentration, 0.32 mm normalizedp-n junction depth, and 10 Kohms p-n junction resistance. Curves 86represents a sample photodetector having a 1.0E18 doping concentration,0.2 mm normalized p-n junction depth, and 150 Kohms p-n junctionresistance. Dark current-voltage characteristics are illustratedcorresponding to photodetector Ge substrate doping concentration,normalized p-n junction depth, and shunt resistance. The reverse leakagecurrent is inversely related to the doping concentration.

Referring now to FIG. 9 a plot of capacitance versus voltage for adiffused junction photodetector formed in accordance with an embodimentof the present invention is shown. Low capacitance photodetectorexamples are illustrated having Ge substrates of approximately 3 ohm-cmresistivity. Three curves 88, 90, and 92 are shown for threephotodetectors having 1 mm, 2 mm, and 3 mm diameters, respectively. Thethree photodetector examples illustrate roll-off capacitance withincreasing voltage.

The present invention provides a photodetector that has low leakagecurrent, (or low capacitance), is less time consuming and relativelyinexpensive to manufacture, and provides versatility in that it may beapplied to many different applications. The present invention is alsoamendable to high volume production.

The above-described apparatus, to one skilled in the art, is capable ofbeing adapted for various purposes and is not limited to the followingsystems: vehicle systems, control systems, communication systems,semiconductor lasers, photodetectors, photodiodes, fiber optic receiverdetectors, solar cells, or other systems or applications that mayutilize a diffused junction semiconductor. The above-described inventionmay also be varied without deviating from the spirit and scope of theinvention as contemplated by the following claims.

What is claimed is:
 1. A diffused junction semiconductor for detectinglight at a predetermined wavelength comprising: a base; and an epitaxialstructure electrically coupled to said base and forming a p-n junctionin said base comprising; at least one diffusion layer electricallycoupled to said base and at least one of said diffusion layerscontributing impurities, which are non-extrinsic elements, in at least aportion of said base to form said p-n junction during growth of saidepitaxial structure.
 2. A semiconductor as in claim 1 wherein said atleast one diffusion layer electrically couples an emitter layer of saidbase to at least one contact.
 3. A semiconductor as in claim 1 whereinsaid at least one diffusion layer comprises a nucleation layer.
 4. Asemiconductor as in claim 1 wherein said at least one diffusion layercomprises a passivating layer.
 5. A semiconductor as in claim 1 whereinsaid at least one diffusion layer comprises a contact layer.
 6. Asemiconductor as in claim 1 wherein at least a portion of said basecomprises p-type material.
 7. A semiconductor as in claim 1 wherein atleast a portion of said base comprises n-type material comprising atleast one of germanium, phosphorus, and arsenic.
 8. A semiconductor asin claim 1 further comprising a passivation dielectric layer coupled tosaid base and preventing carrier recombination of at least a portion ofsaid base.
 9. A semiconductor as in claim 1 wherein said base comprisesa recessed volume comprising an emitter layer.
 10. A semiconductor as inclaim 1 wherein the semiconductor forms a plurality of semiconductordevices and further comprises a channel stop electrically coupled tosaid base and said epitaxial structure, said channel stop preventingreverse bias breakdown between said plurality of semiconductor devices.11. A semiconductor as in claim 1 wherein said impurities areconstituent elements of said at least one diffusion layer.